Synthesis, Crystal Structure and Vibrational Spectra of Barium Cyclotetraphosphate Hydrate Ba 2 (P 4 O 12 )∙3.5H 2 O

Barium tetrametaphosphate hydrate Ba 2 (P 4 O 12 )∙3.5H 2 O was synthesized as a single‐phase crystalline powder starting from an aqueous solution of barium hydroxide and phosphorus pentoxide at 300 K. Ba 2 (P 4 O 12 )∙3.5H 2 O crystallizes in a new structure type in which the Ba 2+ ions form a distorted hexagonal diamond‐like arrangement with the (P 4 O 12 ) 4– anions in the trigonal prismatic voids (Ba 2 (P 4 O 12 )∙3.5H 2 O, C 2/ c , Z = 4, a = 777.3(2), b = 1297.6(2), c = 1346.1(3) pm, b = 95.38(2)°, wR 2 = 0.071, R 1 = 0.018, 1180 reflections, 118 parameters). The vibrational spectra of Ba 2 (P 4 O 12 )∙3.5H 2 O and its thermal behavior up to 720 K are also reported.